The steering apparatus for an automobile is configured as shown in FIG. 10 so that the rotation of a steering wheel 1 is transmitted to the input shaft 3 of a steering gear unit 2 and a pair of left and right tie rods 4, 4 is pushed and pulled with the rotation of the input shaft 3, thereby giving a steering angle to the front wheels. The steering wheel 1 is supported and fixed to the rear end portion of a steering shaft 5, and in a state in which the steering shaft 5 is inserted into the cylindrical steering column 6 supported on the vehicle body in the axial direction, the steering shaft 5 is rotatably supported on the steering column 6. The front end portion of the steering shaft 5 is connected to the rear end portion of an intermediate shaft 8 via a universal joint 7, and the front end portion of the intermediate shaft 8 is connected to the input shaft 3 via another universal joint 9. The example shown in the figure is an electric power steering apparatus being configured to reduce the force required to operate the steering wheel 1 by using an electric motor 10 as assisting force generation source. The front-rear direction stated in this description and all the claims thereof refers to the front-rear direction of a vehicle, unless otherwise specified.
FIGS. 11 and 12 show the electric power steering apparatus described in Patent Document 1 as an example of a more specific structure of an electric power steering apparatus. A steering column 6a is formed of a combination of an inner column 11 and an outer column 12 so that the entire length thereof can be shrunk at the time of a secondary collision, and the steering column 6a is supported on the vehicle body. The steering shaft 5a rotatably supported inside the steering column 6a is formed of a combination of a lower shaft 13 and an upper shaft 14 so that torque can be transmitted and so that the entire length thereof can be shrunk at the time of the secondary collision. The steering wheel 1 (see FIG. 10) is fixed to the rear end portion of the upper shaft 14 protruding from the rear end opening of the outer column 12. A housing 15 is securely connected to the front end portion of the inner column 11, and the front half section of the lower shaft 13 is inserted into the inside of the housing 15. An output shaft 16 is rotatably supported by a pair of ball bearings 17 and 18 on the front side of the lower shaft 13 serving as an input shaft. The output shaft 16 is connected to the lower shaft 13 via a torsion bar 19. The universal joint 7 (see FIG. 10) is connected to the front end portion of the output shaft 16 protruding from the front end opening of the housing 15.
A cylindrical portion 20 is provided at the rear end portion of the output shaft 16. A circumferential groove 21 is provided on the outer peripheral surface of the cylindrical portion 20 around the entire circumference in the circumferential direction. In contrast, on the inner peripheral surface of the cylindrical portion 20, a female stopper portion 22 having a concave-convex shape along the circumferential direction is provided. On the other hand, at the front end portion of the outer peripheral surface of the lower shaft 13, a male stopper portion 23 having a concave-convex shape along the circumferential direction is provided, the outer diameter dimension (the diameter of the circumscribed circle) thereof being smaller than that of the portion near the front end thereof. The male stopper portion 23 and the female stopper portion 22 are in concave-convex engagement with each other so as to be relatively rotatable in a given angular range (for example, ±5 degrees from a neutral state in which the torsion bar 19 is not twisted). Hence, the torsion bar 19 is prevented from being twisted excessively. In the case of the structure shown in the figure, the circumferential groove 21 provided on the outer peripheral surface of the cylindrical portion 20 is disposed on the front side of a stopper engagement region in the axial direction in which the female stopper portion 22 and the male stopper portion 23 are engaged with each other.
The lower shaft 13 is made of steel, a magnetic metal, and a torque detection concave-convex portion 24 having a concave-convex shape along the circumferential direction is provided at the portion near the front end of the outer peripheral surface of the lower shaft 13. A torque detection sleeve 25 made of a non-magnetic metal having electrical conductivity, such as an aluminum alloy, and having a cylindrical shape is disposed radially outside the torque detection concave-convex portion 24. The base end portion of the torque detection sleeve 25 is securely fitted on the cylindrical portion 20. In this state, the edge portion of the base end portion of the torque detection sleeve 25 is clinched to the circumferential groove 21 to position the torque detection sleeve 25 and to prevent the displacement thereof in the axial direction. A plurality of window holes 26, 26 are provided at the portions of the torque detection sleeve 25 positioned radially outside the torque detection concave-convex portion 24. A torque detection coil unit 27 is securely fitted in the housing 15 and is disposed radially outside the torque detection concave-convex portion 24 and the torque detection sleeve 25.
A worm wheel 28 is securely fitted on the portion near the rear end of the output shaft 16. A worm 29 rotatably supported inside the housing 15 is engaged with the worm wheel 28. The electric motor 10 (see FIG. 10) is supported by and fixed to the housing 15, and the output shaft of the electric motor 10 is connected to the base end portion of the worm 29 so that torque can be transmitted.
In the case of the electric power steering apparatus configured as described above, when torque serving as a steering force is applied to the steering shaft 5a by the operation of the steering wheel 1 by the driver, the torsion bar 19 is elastically twisted (in the given angular range) by the amount corresponding to the direction and magnitude of this torque. Accordingly, the positional relationship between the torque detection concave-convex portion 24 and the torque detection sleeve 25 in the circumferential direction is changed, whereby the impedances of the coils of the torque detection coil unit 27 are changed. Hence, on the basis of the changes in impedance, the direction and magnitude of the torque can be detected. The electric motor generates assisting force depending on the detection result of the torque. This assisting force is increased by a worm reducer 30 in which the worm 29 is engaged with the worm wheel 28 and then applied to the output shaft 16. As a result, the force required for the driver to operate the steering wheel 1 is reduced.
On the other hand, when the twisted amount of the torsion bar 19 reaches the upper limit value on one side or on the other side of the given angular range due to the input of large torque from the steering wheel 1 to the steering shaft 5a, the female stopper portion 22 is engaged with the male stopper portion 23 in the circumferential direction. Hence, on the basis of this engagement, part of the torque is directly transmitted from the lower shaft 13 to the output shaft 16. At this time, the cylindrical portion 20 is liable to be twisted as the torque is transmitted.
The torsion rigidity of the cylindrical portion 20 at the axial position where the circumferential groove 21 is provided is made lower than that at the other axial position. The reason for this is that the thickness of the cylindrical portion 20 is made smaller by the depth amount of the circumferential groove 21 at the axial position where the circumferential groove 21 is formed. On the other hand, the portion of the cylindrical portion 20 radially overlapping the stopper engagement region in which the female stopper portion 22 and the male stopper portion 23 are engaged with each other is twisted integrally with the front end portion of the lower shaft 13 on which the male stopper portion 23 is formed, at the time of the above-mentioned torque transmission. Hence, at this time, the front end portion of the lower shaft 13 serves as a reinforcement member, whereby the torsion rigidity of the portion of the cylindrical portion 20 radially overlapping the stopper engagement region in which the female stopper portion 22 and the male stopper portion 23 are engaged with each other is improved. However, in the case of the above-mentioned conventional structure, the circumferential groove 21 is not radially overlapping the stopper engagement region in which the female stopper portion 22 and the male stopper portion 23 are engaged with each other. For this reason, the torsion rigidity of the portion of the cylindrical portion 20 provided with the circumferential groove 21 on the outer peripheral surface thereof is not improved particularly at the time of the above-mentioned torque transmission. Hence, in the case of the above-mentioned conventional structure, the thickness of the cylindrical portion 20 is required to be made large to some extent to secure torsion rigidity at the portion of the cylindrical portion 20 provided with the circumferential groove 21 on the outer peripheral surface thereof. As a result, when the respective components for torque detection and the peripheral portions thereof are made smaller in diameter and lighter in weight, it is difficult to make the cylindrical portion 20 smaller in thickness.